1. Field of the Invention
This invention relates to a propeller assembly and a method for manufacturing propeller assemblies. More particularly, this invention relates to a through-the-barrel exhaust marine propeller assembly with replaceable blades.
2. Background Discussion
Since about 1850, marine propellers have been utilized in the propulsion of ships. The marine propellers traditionally have been cast or forged of metal with the hub and blades formed as a single unitary part.
One or more of the propeller blades in a unitary propeller assembly, especially those used in a marine environment, often become damaged so as to require replacement of the entire propeller assembly. When a marine propeller strikes an object below the surface of the water one or more of the blades may become bent or chipped causing a reduction in the efficiency of propulsion and, possibly, degradation to the driving means due to imbalance of the blades. The requirement for replacement of the entire propeller assembly especially for those situations where only a single blade has been slightly damaged is very frustrating to boat owners, especially in view of the expense associated with the purchase of a unitary propeller assembly.
In an attempt to avoid the problems associated with replacement of entire propeller assemblies, attempts have been made to provide a propeller assembly with replaceable blades. U.S. Pat. Nos. 123,274; 752,670; 1,363,660 and 3,132,698 are illustrative of such attempts to provide a propeller with replaceable blades. These references also reveal the use of dovetail connections between the hub and blades.
The prior art attempts to provide replaceable propeller blades suffer from numerous drawbacks such as:
1. insufficient support against distortion of the blade upon contact with an object;
2. complicated methods of attachment of the blade to the hub which increased expense and, in some instances, assembly/disassembly time;
3. susceptibility of the attachment means to corrosion (especially in salt water environments) which increases the time for disassembly;
4. the likelihood of the hub becoming damaged or worn upon one or more of the blades becoming damaged.
5. the inability to protect the driving means by absorbing or dampening the shocks which originate with a propeller blade upon contact with an object; and
6. the inability to avoid harmonic frequencies and vibrations being passed from the driving motor to the propeller assembly.
The aforementioned U.S. Pat. No. 3,132,698 illustrates some of the problems associated with prior art propeller blade assemblies. U.S. Pat. No. 3,132,698 reveals a propeller blade with a dovetail that is designed for sliding into a slot. The connection between the blade and the hub is such that the blade, upon contact with an object, would have a tendency to twist within the slot. Such twisting would damage the flanges forming the dovetail slot or, in some instances, cause sufficient wear in the dovetail slot or dovetail to cause propeller wobbling or imbalance. Furthermore, such an assembly as that in U.S. Pat. No. 3,132,698 would require, especially for larger horse power motors, that the hub be formed of a high strength material which is not susceptible to distortion or wear. In addition to the expense associated with a high strength material such as stainless steel, the high strength material is also likely to create a relatively heavy hub assembly which can cause increased wear in the driving means.
The aforementioned U.S. Pat. No. 1,363,660 features a plurality of radially extending screws fastening the propeller blade to the hub. Such a manner of attachment is likely to avoid blade distortion, but only at the expense of an increase in the time for assembly and disassembly. The manner of attachment in U.S. Pat. No. 1,363,660 is also likely to increase the chance of corrosion or rusting causing difficulty in disassembly.
U.S. Pat. Nos. 3,764,228; 3,876,331; 4,417,852 and 4,930,987 illustrate marine propeller assemblies having replaceable blades and exhaust ports through the hub. U.S. Pat. No. 3,876,331 discloses a structure which avoids the use of threaded couplings so as to avoid freeze up problems. The attachment method of U.S. Pat. No. 3,876,331 presents a relatively large number of components which are susceptible to breaking or loss. In addition, securement of the blades with collars or rings present the problem of low blade impact survivability or durability due to factors such as the attachment being away from the center of the blade. U.S. Pat. Nos. 3,764,228 and 4,930,987 also rely on ring or collar connections and thus suffer from the same deficiencies in attachment durability or survivability.
The state of the prior art for such exhaust through propeller assemblies is also such that the hubs are formed of casted stainless steel to achieve sufficient strength. Stainless steel hubs are relatively heavy and have a tendency to quickly wear out the moving parts of the driving means. Furthermore, the requirement for drafts in the casting technique reduces the maximum exhaust gas passageway size resulting in a drop in propulsion efficiency. The exhaust passageway size limitations imposed by draft angle requirements in the casting process also restricts the range of suitable motor sizes which can be used with the casted blade assembly.
The aforementioned U.S. Pat. No. 4,417,857 features a hub which is placed in direct contact with the spline of the driving means. This arrangement results in the forces which develop during blade impact being passed directly to the driving shaft and the remainder of the driving means. In addition, the driving mean's vibrations and harmonic frequencies are not absorbed and therefore are passed to the propeller blade so as to reduce propulsion efficiency.
The prior art requirement for the hubs to be formed in a casting process results in high manufacturing expense in materials, time consumption, labor and extensive finishing requirement.